Steels for high-temperature mechanical strength include martensitic steels that can be used to around 550° C., non-oxidizing austenitic steels containing a hardening intermetallic phase precipitation, which can be used to around 650° C. Alloys of nickel or cobalt are also used, generally hardened by intermetallic precipitation.
Non-oxidizing austenitic steels for high-temperature mechanical strength, such as the steel with reference no. 1.4980, according to European standard EN 10269, also referenced as AlSi 660 according to the standard ASTM A453, are frequently used in bolt and screw manufacturing and forged parts, in particular in fasteners for automotive exhaust elements, such as turbocompressors or exhaust pipes. They are also found, in the form of drawn wires, in mesh for mechanical trapping in exhaust gas catalytic converters. Applications for these steels are also known in the area of springs that can be used at high temperature or exhaust hoses made up, on one hand, of rolled tubes—welded then crimped, and on the other hand, of metal wire mesh sheathing.
The composition of the steel AlSi 660 has a moderated chromium content, on the order of 15%, about 1% molybdenum, 0.3% vanadium. The austenitic character, necessary for
high-temperature strength, is insured by a massive addition of nickel, i.e., on the order of 24%.
The hardening and the resistance to creep are insured by an addition of around 2% titanium, which is combined between 600° C. and 750° C. with one part nickel to form intermetallics of the type Ni3Ti. The steel composition can also contain elements such as Mo, V, Al which also contribute to hardening and high-temperature strength by substituting atoms of titanium in the Ni3Ti compound.
The disadvantages of this steel are, in particular:    increased costs, particularly due to the significant nickel content,    difficulty in manufacturing since, at the time of pouring, there are segregation formations which, unless specific precautions are taken, cause cracks in continuous pouring or at the time of hot rolling; as a result, it is necessary to use a costly manufacturing process involving remelting with grinding of the semi-finished products and increased inspections of the finished products.
To reduce the segregations, the silicon must be limited to a content of less than 0.3%, carbon to a content less than 0.050%, copper to a content lower than 0.5%, sulfur to a content less than 0.002%, phosphorous to a content less than 0.025%, lead to a content less than 0.0005%, etc. These limitations represent the additional costs of manufacturing at the steel plant.                difficulty in rolling since the segregations greatly lower the burning point. Because of this, rolling must not be carried out above around 1150° C. in order to avoid the formation of serious defects, e.g. hot cracks. Taking into account the increased yield stress of the alloy below this temperature, the rolling cannot be carried out except on certain particularly robust systems. In addition, the rolling speed must be reduced in order to avoid any reheating above the burning point.        a limitation in the resistance to oxidizing and corrosion at high temperature because of the low amounts of chromium and silicon, under particularly intense exposure conditions, e.g. in exhaust lines.        difficulty in machining parts, particularly because of the small amount of sulfur.        difficulty in welding, especially in the case of AlSi 660 sheet metal welded to itself, with or without a supply of wire of the same alloy, since a great tendency to fissuring at high temperature is observed.        
In the family of austenitic steels for high-temperature mechanical strength, hardened by intermetallic nickel-titanium precipitation, the following are known:    the steel AlSi 660 referenced above,    an IMPHY patent No. FR 94 14 942 that describes the following composition:    Ni: 16% to 25%; Cr: 16% to 18.5%; Ti: >1%; Mn: 0% to 2%.    a NIPPON KOKAN patent JP 62267453 describing the following composition: C<0.01%; Ni: 10% to 18%; Cr: 13% to 20%; Ti: >1.5%; Mn: 0% to 2%.
Theoretical knowledge of the phases present at the time of solidification or in solid phase, in the quaternary alloys Fe—Cr—Ni—Ti remains incomplete. This was published by V. RAGHAVAN in 1996 in “Phase diagrams of quaternary iron alloys,” ed. The Indian Institute of Metals, pages 374 to 380. The range analyzed does not extend to compounds containing more than 1.7% Ti.
We have noted that the main difficulties encountered with the steel AlSi 660 result from its solidification mode, which proves to be direct solidification in austenitic form, in contrast to the majority of non-oxidizing austenitic steels, which solidify in ferrite, which then transforms to austenite at lower temperature.
The alloy according to the IMPHY patent, with limited chromium content, has austenitic solidification, as we will demonstrate in the following. Thus it is subject to the problems in pouring and rolling that are connected with segregations.
The composition of the alloy according to the NIPPON KOKAN patent shows a low amount of nickel mixed with a chromium content between 13% and 20%. The nickel content expresses itself inadequately to insure hardening and an effective creep resistance at 650° C. and above. In addition, the very small amount of carbon, less than 0.010% makes it unsuitable for manufacturing in air. In all cases, it probably does not solidify to ferrite.